Finally, if we narrow the question into, why do we not find in some instances every intermediate form between any two species? the answer may well be that the average duration of each specific form (as we have good reason to believe) is immense in years, and that the transition could, according to my theory, be effected only by numberless small gradations; and therefore that we should require for this end a most perfect record, which the foregoing reasoning teaches us not to expect. It might be thought that in a vertical section of great thickness in the same formation some of the species ought to be found to vary in the upper and lower parts{323}, but it may be doubted whether any formation has gone on accumulating without any break for a period as long as the duration of a species; and if it had done so, we should require a series of specimens from every part. How rare must be the chance of sediment accumulating for some 20 or 30 thousand years on the same spot{324}, with the bottom subsiding, so that a proper depth might be preserved for any one species to continue living: what an amount of subsidence would be thus required, and this subsidence must not destroy the source whence the sediment continued to be derived. In the case of terrestrial animals, what chance is there when the present time is become a pleistocene formation (at an earlier period than this, sufficient elevation to expose marine beds could not be expected), what chance is there that future geologists will make out the innumerable transitional sub-varieties, through which the short-horned and long-horned cattle (so different in shape of body) have been derived from the same parent stock{325}? Yet this transition has been effected in _the same country_, and in a far _shorter time_, than would be probable in a wild state, both contingencies highly favourable for the future hypothetical geologists being enabled to trace the variation. {323} See _More Letters_, vol. I. pp. 344-7, for Darwin's interest in the celebrated observations of Hilgendorf and Hyatt. {324} This corresponds partly to _Origin_, Ed. i. p. 294, vi. p. 431. {325} _Origin_, Ed. i. p. 299, vi. p. 437. CHAPTER V GRADUAL APPEARANCE AND DISAPPEARANCE OF SPECIES{326} {326} This chapter corresponds to ch. X of _Origin_, Ed. i., vi. ch. XI, "On the geological succession of organic beings."